1. Field of the Invention
This invention relates to a flow rate control valve system for controlling the flow rate by controlling the valve plug position by converting the rotation of a stepping motor into a linear movement.
2. Background of the Invention
A flow rate control valve system of the sort disclosed by, for instance Japanese Patent Application (OPI) No. 194658/84 is constructed so that a valve plug is fixed to the front end of the output shaft of a stepping motor contained in a mechanism for converting its rotation into a linear movement. Alternately, such a mechanism for converting the rotation of a stepping motor into a linear movement is provided by screwing and fitting the externally threaded linear output shaft, which is allowed to move only in the axial direction, into the threaded hollow rotary shaft of the stepping motor. When the mechanism is utilized to drive the flow control valve, the front end of the linear output shaft is fixed to the valve plug and the output shaft is moved in the axial direction proportionally to the amount of rotation of the stepping motor to control the flow rate by controlling the position of the valve plug relative to the valve seat.
An example of the latter structure is disclosed by Kobayashi et al in U.S. Pat. No. 4,381,747. In this structure, the output shaft of the stepping motor is fixed to the valve plug and is prevented from rotating by an arm affixed to the output shaft and sliding in an axial slot of the motor housing. The output shaft is threaded externally to internal threads of the motor rotor. Thereby, the turning rotor axially moves the output shaft. However, there results a long projection of the output shaft.
Since the rotary shaft of the stepping motor thus constructed holds the output shaft linearly moving therein, the diameter of the rotary shaft tends to become large and thus bearings for bearing the housing tend to become large and costly. Moreover, the output shaft must also be pivotally supported and this makes the construction of the bearings for bearing both shafts within the housing not only complicated but also large. In addition to the above problem, the total weight of the portions linearly driven and the inertia increase because the valve plug and the output shaft are linearly moved together when the valve plug is driven by the output shaft, makes it extremely difficult to implement highly - responsive, high-speed and accurate flow rate control.